1. Field of the Invention
The present invention relates to a sheet stacking device, a sheet feeding device, and an image forming apparatus including the same.
2. Description of the Related Art
In a case of forming an image on an envelope whose sealing flap is folded and if a plurality of such envelopes is set while aligning with each other within a feed cassette, a height (thickness) of the envelopes on a side of a region where the flaps exist within the feed cassette becomes higher than a height of the envelopes on a side of a region where there is no flap. The more the number of stacked envelopes, the greater the difference of the heights becomes in this state, and an uppermost envelope is inclined remarkably.
FIG. 15 shows a state in which a plurality of envelopes P is set within a storage box 506 of the prior art sheet feeding device. As shown in FIG. 15, if the plurality of envelopes P is set such that the flaps are positioned in a direction orthogonal to a sheet feeding direction (a direction vertical to the sheet of FIG. 15), the envelopes P thus set are inclined. At this time, there is a case where a region PL side where there is no flap (referred to appropriately as a ‘flap-less region PL’ hereinafter) of an uppermost envelope P is positioned below a wall surface 506a downstream in the sheet feeding direction of the storage box 506. If the envelope is fed in this state, there is a problem that the flap-less region PL of the envelope P abuts against the wall surface 506a downstream in the sheet feeding direction and the envelope P cannot be fed. Even if the flap-less region PL of the uppermost envelope P is not positioned below the sheet feeding downstream wall surface 506a of the storage box 506, there is a possibility of causing feeding failure because an area of contact with a feed roller 503 is reduced if the envelope P thus set is inclined.
With regard this problem, Japanese Patent Application Laid-open No. Hei. 11-35175 has proposed a sheet feeding device provided with an envelope pressing roller above set-up envelopes and configured to prevent an uppermost surface of the set-up envelope from inclining by pressing a region of the envelope where there is a flap, i.e., a bulge, by the envelope pressing roller.
However, because the sheet feeding device disclosed in Japanese Patent Application Laid-open No. Hei-11-35175 is configured to absorb a difference in a vertical direction between the bulge of the flap part of the sheet bundle (bundle of envelopes) and parts other than the flap part only by pressing by the roller, the set-up envelopes are tightened closely with each other and a friction force between the envelopes increases. Therefore, a conveyance load of the feed roller increases, possibly causing a feeding failure and others due to a slip of the feed roller.